Well, one of their primary goals for GTDH was certainly to make the physics more realistic. (As seen in one of the screenshots previously posted). So It is safe to say that by the time GT5 comes out, we'll have an idea of how realistic they are going to be.
I have only one concern. Since PD included in GTHD a lot of features that were supposed to come in the GT5 version, do you think there will be people saying: "meh, I got GTHD, it has pretty much what I wanted from GT5 and I've already spent some money on it...I see no reason why I should throw that money away by paying more money for an almost identical game to this one" Amd therefore GT5 sales would go downhill.
Trust me, people are very conformist. I know people who have neglected to buy GT4 just because GT3 was very realistic in comparison to GT2, and it was very similar to GT4.
Edit: Perhaps I should've posted this on the other thread
Ciao!